The healthy heart produces regular, synchronized contractions. Rhythmic contractions of the heart are normally controlled by the sinoatrial (SA) node, specialized cells located in the upper right atrium. The SA node is the normal pacemaker of the heart, typically initiating 60-100 heart beats per minute. When the SA node is pacing the heart normally, the heart is said to be in normal sinus rhythm (NSR).
If heart contractions are uncoordinated or irregular, the heart is denoted to be arrhythmic. Cardiac arrhythmia impairs cardiac efficiency and can be a potential life threatening event. Cardiac arrhythmias have a number of etiological sources including tissue damage due to myocardial infarction, infection, or degradation of the heart's ability to generate or synchronize the electrical impulses that coordinate contractions.
Bradycardia occurs when the heart rhythm is too slow. This condition may be caused, for example, by delayed impulses from the SA node, denoted sick sinus syndrome, or by a blockage of the electrical impulse between the atria and ventricles. Bradycardia produces a heart rate that is too slow to maintain adequate circulation.
When the heart rate is too rapid, the condition is denoted tachycardia. Tachycardia may have its origin in either the atria or the ventricles. Tachycardias occurring in the atria of the heart, for example, include atrial fibrillation and atrial flutter. Both conditions are characterized by rapid, uncoordinated contractions of the atria.
Ventricular tachycardia occurs, for example, when a pulse is initiated in the ventricular myocardium at a rate more rapid than the normal sinus rhythm. Ventricular tachycardia can quickly degenerate into ventricular fibrillation (VF). Ventricular fibrillation is a condition denoted by extremely rapid, nonsynchronous contractions of the ventricles. The rapid and erratic contractions of the ventricles cannot effectively pump blood to the body and the condition is fatal unless the heart is returned to sinus rhythm within a few minutes.
Implantable cardiac rhythm management systems have been used as an effective treatment for patients with serious arrhythmias. These systems typically comprise circuitry to sense signals from the heart and a pulse generator for providing electrical pulses to the heart. Leads extending into the patient's heart are connected to electrodes that contact the myocardium for sensing the heart's electrical signals and for delivering pulses to the heart in accordance with various therapies for treating the arrhythmias described above.
Pacemakers may be incorporated into cardiac rhythm management systems to deliver pace pulses to the heart. Pace pulses are low energy electrical pulses timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency. Pace pulses may be intermittent or continuous, depending on the needs of the patient. There exist a number of categories of pacemaker devices, with various modes for sensing and pacing the heart. Single chamber pacemakers may pace and sense one heart chamber. A typical single chamber pacemaker is connected to a lead extending either to the right atrium or the right ventricle. Dual chamber pacemakers may pace and sense two chambers of the heart. A typical dual chamber pacemaker is typically connected to two leads, one lead extending to the right atrium and one lead to the right ventricle.
Pacemakers may be used to provide pacing pulses to both the left ventricle and the right ventricle. This type therapy may be used, for example, to coordinate ventricular contractions when a patient suffers from congestive heart failure (CHF). Congestive heart failure is a condition wherein the muscles of the heart deteriorate, causing the heart muscle to enlarge. Enlargement of the heart causes the contractile impulses to travel more slowly, resulting in asynchronous contractions of the left and right ventricles and reduced pumping efficiency.
Pacemakers can be programmed to provide pace pulses to the heart on demand or at a fixed rate. When a pacemaker paces the heart at a fixed rate, the pacemaker provides pace pulses to the heart without taking into account the heart's spontaneous action. In contrast, pacemakers may sense the spontaneous activity of the heart and provide pace pulses synchronized to the spontaneous activity.
For example, a single chamber ventricular pacemaker may sense and pace a ventricle. The pacemaker senses ventricular activity and provides a pace pulse to the ventricle if no spontaneous activity is sensed. If the pacemaker senses spontaneous activity, the pacing pulse is inhibited. In this example, where the pacemaker senses the ventricle, paces the ventricle and inhibits the ventricular pace pulse upon sensing a spontaneous R-wave, the pacemaker mode is denoted VVI. Alternatively, a single chamber pacemaker may sense and pace the atrium. In the case where the pacemaker senses the atrium, paces the atrium and inhibits the atrial pace pulse upon sensing a spontaneous P-wave, the pacemaker mode is denoted AAI.
A dual chamber pacemaker may be capable of sensing and pacing both the atrium and ventricle. The dual channel pacemaker is capable of using pace pulses to synchronize atrial and ventricular activity. If spontaneous cardiac activity is detected in the atrium or the ventricle, pacing pulses may be triggered or inhibited. When the pacemaker paces and senses both chambers and can trigger or inhibit pace pulses based upon sensed signals, for example, the pacemaker mode is denoted DDD. Various other configurations involving providing or inhibiting pace pulses based upon sensed cardiac events using dual or single chamber pacemakers are known in the art.
Rate adaptive pacemakers provide pacing at rates responsive to the patient's metabolic activity. Changes in metabolic activity may reflect exercise or non-exercise related changes, such as stress or excitement. The level of metabolic activity may be determined by sensing motion, respiratory rate, QT interval, venous oxygen saturation, temperature, or other patient conditions, for example. The pacemaker automatically adjusts the pacing rate to accommodate the sensed changes in the patient's condition.
Implantable cardioverter/defibrillators (ICDs) have been used as an effective treatment for patients with serious cardiac arrhythmias. For example, ICDs are capable of delivering high energy shocks to the heart, interrupting the ventricular tachyarrhythmia or ventricular fibrillation and allowing the heart to resume a normal rhythm. ICDs may include pacing functions described above as well as a cardioversion/defibrillation capability.